The invention is based on a process for speeding up the braking intervention in the traction control mode of a hydraulic brake system for motor vehicles, and a hydraulic brake system for motor vehicles with a traction control system, set forth hereinafter.
Hydraulic brake systems with traction control systems of this kind have already been proposed (German Patent Application P 41 07 978.7; German Utility Model Application G 91 02 352.1), in which the traction control systems have at least one self-aspirating pump for aspirating pressure fluid through an intake line and a charging valve, which is disposed in the intake line and is normally closed, and/or through a negative pressure protection valve assigned to at least one wheel brake and a shutoff valve for furnishing pressures for carrying out the traction control. Since the pressure fluid has high viscosity at low temperatures, braking interventions in the traction control mode can be attained at low temperatures by means of the pumps in the starting phase only disadvantageously slowly. It was therefore proposed that the pressure fluid to be delivered to the pump in these hydraulic brake systems be heated, to reduce the viscosity of the pressure fluid, when the ambient temperatures were below -5.degree. C., for instance. In accordance with one proposal, this is done by repeatedly pumping the pressure fluid through a pressure limiting valve that is operative in the traction control mode, and in accordance with the other proposal by electrically heating the intake line, which leads from a supply tank to the pump. Unavoidable cooling of the traction control system, or a sharp drop in the ambient temperature, causes the pressure confined in these hollow spaces to contract more markedly than the hollow spaces themselves. The result is negative pressures in the pressure fluid, so that the pressure fluid may perhaps shed gas bubbles. Particularly if such gas bubbles coalesce into larger bubbles over the course of several hours, they impede pumping processes, making braking interventions in the traction control mode disadvantageously slow.